Get the Gold cup in all 50cc , 100cc and 150cc cups
You have to win first place on 150cc Special Cup
win 2,700 races
Win 150CC special cup
You must win the Special Cup in 1st place under 100cc and then it should have your Mii in the character selection area.
win in 1st or 2nd in the cup
Win 50cc Star Cup
there is no cup to unlock Diddy Kong you must go on the same account like 180 times (or more, i don't know) and it'll tell you that you unlocked Diddy Kong.
You win baby daisy on the 50cc not on a specific cup you have to gain star ranking for Mushroom, Flower, Star, and Special Cups
to get mii outfit 2 you have to unlock all expert ghots on the time trials
Play on all 32 courses in Time Trials mode or win 1,000 Wi-Fi online matches.
Win the Leaf cup with a trophy (to unlock the Leaf cup, win the Bannana cup and the Shell cup with a trophy)
you have to win the 100cc cup with at least a "One Star". a one star is doing you're best like not using Bullet Bills and staying in first all the time